Abrasive water jet cutting is a known method and applied for cutting numerous materials including composite materials.
The principle of this cutting method is recalled hereinafter. A mobile cutting head is fitted with a nozzle by which a water jet is ejected with a very small diameter (0.1 to 0.5 mm), at a very high speed, and mixed with abrasive particles. This type of jet is capable of cutting through any type of material. The nozzle is located a few millimetres above the part to be cut. By moving the head, the water jet cuts the parts to the desired shape. The plate being cut is generally placed on a grating above a pool, whose purpose is to absorb the jet as soon as it has crossed through the part and to reduce noise levels.
In the prior art, when the jet reaches the pool after having crossed through the part, a significant level of boiling is produced. If the plate being cut is light, for example if it is comprised from composite material with a low thickness, this boiling can raise the cut parts that float and become blocked between the jet and the plate remaining to be cut.
In other examples, when cutting very thin parts, the stress relief within the material being cut can lead to a plate “rolling around itself” phenomenon, which can also cause collisions with the nozzle.
In order to avoid this type of collision, a simple solution consists in sufficiently spacing the parts in such a way that even if they become raised, they cannot come into contact with the nozzle. Typically, on planar cutting machines capable of cutting plates measuring 3 m2 to create a range of complex parts, a minimum distance of 20 to 40 mm between the parts is sufficient to eliminate almost any risk of collision. The disadvantage of this principle is that the nesting compactness of the parts becomes degraded, which creates a particular problem for high cost materials such as aeronautic plates made out of composite materials. With this type of spacing, the compactness (surface area of the part/surface area of the whole plate) scarcely exceeds 60%.
In the prior art, solutions are known to correct this problem, however none of these solutions provide a truly satisfactory result. The following make up part of these known solutions:
A first solution consists in covering the plate being cut with another, heavier and generally thicker plate. This other plate thus limits the possibility of cut parts floating. However, in addition to the fact that this other plate leads to the consumption of “martyr plates”, this solution reduces the quality and precision of the cut. Indeed, the jet is thus located further away from the parts being cut. As this jet is divergent and loses its coherence after a relatively short distance, this spacing of the jet creates a variation in dimension and an undercut edge shape. This solution therefore reduces the precision of the cut and the quality of the cut parts.
Other solutions exist based on vacuum systems using, for example, air vents positioned on the grating to hold the cut parts. This type of device is difficult to implement for plates with large dimensions (several m2) in which a range of small parts are cut. Indeed, the jet must not deteriorate the holding devices. The position of the latter must therefore be taken into account in the cutting programme. This restriction reduces the nesting compactness and makes the implementation operation difficult if said nesting operations are performed “with the flow” within the scope of a stressed flow production.
Another solution is given by JP2005230994, which describes a water jet cutting method for semi-conductors aimed at producing electronic chips, i.e. parts with very small dimensions. In this embodiment, a double-sided polyethylene adhesive film is bonded underneath the plate being cut. On the other side, it touches a mesh comprised of “piano wire” with a diameter of between 0.1 and 0.5 mm. Then, a single-sided polyethylene adhesive film is bonded to the mesh and to the first film. The two films thus assembled with the internal mesh are placed under the substrate during the cutting operation. In jet cutting methods, the upper side is considered as the side exposed to the water jet. During the cutting operation, the jet crosses through the plate, thus cutting the latter into small squares. However, in this implementation, the jet does not manage to cut the piano wires, in particular as it has lost energy when crossing through the plate. Thus, the cut elements remain connected to each other via the adhesive from the parts of the film connected to the mesh that has not been cut, and can therefore be easily recovered. Such a device is difficult to transpose onto parts with large dimensions, taking into account the preparation time required in addition to the fact that the thickness of the parts concerned by the disclosed embodiments is insufficient in reducing the power of the jet in a significant manner.
Another solution, applied in other technologies for cutting a range of parts on the same plate consists in leaving fasteners or “small bridges” between the cut parts. These fasteners are themselves cut or broken by various means after the cutting process. The advantage of such a solution is that it enables the entire plate to preserve a certain level of mechanical cohesion. This enables the plate to be easily handled, in particular with regards to removing all of the cut parts from the cutting machine table in a single manoeuvre. This is particularly advantageous if this operation is to be automated. The notion of preserving this cohesion of the original plate is also efficient in limiting the deformations connected to stress relief.
However, this technique has major disadvantages when applied within the scope of a water jet cutting technology, and more particularly when involving the cutting of parts made out of composite materials. A cutting operation cannot be stopped and subsequently restarted, because the water jet cutting operation is created by two simultaneous effects:                The high pressure of the water (which creates a high speed jet output)        The presence of an abrasive within the jet.        
Without an abrasive, the high performance composite materials cannot be cut. Moreover, the application of a very high pressure water jet without abrasive to the surface of a composite material of this type produces a significant level of delamination around the jet impact area.
The abrasive is mixed with the jet via a venturi device which sucks the abrasive thanks to a vacuum created by passing the jet at a high speed in a “barrel”.
The jet cut off method is therefore as follows:                abrasive feed cut off        progressive decrease in pressure (800 bars)        jet cut off        
The reactivation of the cutting jet uses the reverse method:                start-up at low pressure (800 bars)        progressive increase in pressure (up to 2,500-3,000 bars)        abrasive feed.        
Even if the progressive increases and decreases in pressure occur over short periods of time lasting approximately one second, the interaction of the high pressure jet without abrasive with the material presents a certain risk of causing delamination. This is why; in particular, all of the water jet's first contacts with the material generally occur outside of the areas of the parts, in areas making up the “skeleton” of the range of parts, which is later thrown away. Moreover, the cutting speed is approximately equal to 4 to 6 m/min according to the machines, types of materials cut and their thickness. During a “cutting/reactivation” cycle at this speed, the cutting head travels 8 to 10 mm, which involves a large area and creates a “large fastener”. The cutting can be moved to the skeleton on either side of the fasteners. This therefore significantly extends the cutting time. Another possibility includes boring areas on either side of said fasteners with a drill before the jet cutting process. This requires a drill head to be fixed onto the machine, in addition to substantially extending the cutting time.
However, in all of these examples, the characteristics of the materials are such that the fasteners thus left must be sawn and filed down: they cannot be broken as the fastener, remaining relatively resistant, could cause delamination within the parts. The range of parts must therefore be processed manually, which reduces the level of productivity of these operations for cutting a range of parts.
These solutions from the prior art are not without disadvantages, in particular due to wasting materials by increasing the surface area of discarded products and increasing the cutting time for a plate.